There exists a continuing need for a portable source of energy. Bio-fuel cells convert hydrogen-rich fuel, such as hydrogen or methanol, into electric current. In a bio-fuel cell, electrochemical reactions may take place inside a chamber containing two electrodes that are separated into a first cell and a second cell by a membrane that allows for selective transport of ions. Hydrogen molecules are oxidized in the first cell at a relatively negatively charged conductive anode to generate protons and electrons that may be conducted through the anode. Catalysts may be used to facilitate reduction oxidation of the hydrogen. The electrons can travel through an electrical circuit when an electrical potential exists between the anode and cathode. The protons in the first cell can diffuse through the membrane to the second cell, which contains a relatively positively charged cathode electrode. Subsequently, oxygen molecules are reduced at the cathode electrode where oxygen may be combined with electrons and protons to form water.
Hydrogen-consuming fuel cells may be extremely efficient, but pure hydrogen may require storage in pressurized tanks and other precautions. One common type of micro fuel cell is based on methanol. Methanol is a liquid fuel with a high energy density and is plentiful and inexpensive. However, methanol fuel cells do not always work at high efficiency. Methanol may also block the reactions that form water.
Fuel cells may use enzymes to carry out the reactions. For example, one or more enzymes may be localized in the region of separate electrodes to ensure that the proper reactions occur at the desired location. For example, the enzyme glucose oxidase may be maintained in the region of an anode in the presence of glucose to oxidize glucose thereby converting the glucose sugar molecule to gluconolactone and a pair of hydrogen ions. Hydrogen ions then migrate through a semi-permeable membrane to the cathode. Similarly, an enzyme called laccase may be maintained in the region of the cathode. In the presence of oxygen, laccase can combine the protons with oxygen and electrons to produce water. However, the activity of laccase is pH dependent. Since laccase enzymes typically work best in environments much more acidic than the pH 7.0, laccase-based fuel cells do not produce a high yield of power.
Moreover, many existing chemical and metal batteries present numerous challenges to manufacture as well as challenges to develop disposal practices that are not environmentally detrimental. Thus, there is a need for bio-batteries that are re-usable, disposable, and environmentally safe. There is also a need for bio-batteries that weigh less and have an increased power output and battery life.